some limits would be how closely you can measure your actual device material properties and dimensions
most speaker driver materials will vary in audibly important damping charateristics that won't be found in material handbooks, will be processing dependent to a large degree - can you test samples from the lots of materials used by the manufacturer for your specific drivers - or do you rely on NDT tests on the finished device?

at a more practical level Klippel is one source of info, test equipment: Introduction

I know the general answer to my question and jcx points out things to help understand better..

What about thoughts on how much of the overall sonic characteristics of a great design is beyond the measured numbers .. I say a fair amount and this is the biggest difference between all well designed speakers.. Am I right?

I think we know all of the causal relationships, too, if we want to badly enough. But we draw the line, on both measurements and physics models, at some point of diminishing returns, i.e. "that's close enough".

I dont really think the question should be is there something missing from the measurements. What needs to be resolved is, what order(s) of distortion, at what relative level(s), and at which frequency(s) is/are pleasing/displeasing.

What about thoughts on how much of the overall sonic characteristics of a great design is beyond the measured numbers .. I say a fair amount and this is the biggest difference between all well designed speakers.. Am I right?

I think for what we currently know: we can pretty much measure *most* of what does (or does not) make fore a better measured loudspeaker (..a more linear loudspeaker that is).

There are two over-riding issues however that substantially "muddy the waters":

1. Just because you can measure doesn't mean you can correlate that measurement with an effect. Most mechanical effects - sure, but most subjective responses.. not so much (at least not currently). The issue I brought up on "depth" perspective (in the Orion/Behringer thread), seems to be a good example. People know what "depth" is - it's self-explanatory. IF system "A" provides enough of a difference from "B" in the portrayal of depth - then it starts becoming an obvious effect, but point to that on any graphical display of Impulse Response? Not really.

2. It's particularly bad when an objectively better design may well be incorrect for subjectively better stereo reproduction. Stereo is just plain *flawed* - designing the "best loudspeakers ever" may still result in overall mediocrity for any given listener. Honestly, because of this fact alone I don't think anyone will *ever* create a "perfect" stereo loudspeaker system..

Still, both aspects 1 & 2 are interesting impediments to attempt to overcome - and it tends to make the hobby that much more interesting. Some tend to scoff: "there's no *magic*!". But when no one can sufficiently provide an objective answer to an effect, then yeah - it's pretty much "magic" until it can be.

Stereo is just plain *flawed* - designing the "best loudspeakers ever" may still result in overall mediocrity for any given listener. Honestly, because of this fact alone I don't think anyone will *ever* create a "perfect" stereo loudspeaker system..

Oh yes, also a human is flawed
It has been misunderstood; recordings were made to make it eternal !
as we need to reproduce an event that happened in another time, in another place. So a recording has to bring to our ears what has happened then, which is a sequence in time and space...in a relatively very slow motion.
The cracks in the system allow so many variants that may corrupt this slow motion. Indeed what we hear is the processment of it all, nothing else.
So what we call stereo is just a double sequencing of a reality captured.
Making it synchronized to our expectations....so also the human being with its culture, emotivity or just brain subconcious activity in differentiating the arrival time and then elaborate, resulting ever time as a new event....like reading a book twice, or watching a landscape, but there's a little conceptual difference : you are hearing to something happened in another time, but that thing is happening again at your place